Prior art processes for preparing thermoplastic polymer-biofiber composite materials containing poly(vinylchloride) (PVC, or hereinafter simply called "polymer") and a biofiber such as a wood fiber teach the pre-blending and single addition of all components to extruders. Melt processes for such composites are disclosed in Deaner et al., U.S. Pat. No. 5,827.607 and Deaner et al., U.S. Pat. No. 5,518,677. Process equipment manufacturers routinely suggest that intimate mixing and throughput can be more effectively optimized by independently introducing polymer and biofiber at different points in the process. For example, a publication in the Proceedings of the Fourth International Conference on Wood Fiber-Plastics Composites; May 12-14, 1997; published by Forest Products Society, Madison, Wis. 53705-2295; entitled Processing Wood-Polymer Composites on Twin Screw Extruders, authored by Augie Machado and Charlie Martin of American Leistritz Extruder Corp., teach that wood fiber should be added to fluxed (melt/sheared) polymer. Fluxed polymer is sheared polymer in melt form, said form arising from both mechanical action and heat input. The input of shear causes fusing to form a viscous liquid (fluidic), visco-elastic polymer mass of relatively low viscosity compared to unheated or unsheared polymer. Mixing with polymer at low viscosity helps to avoid shear stresses capable of breaking wood fibers. Similarly, Krupp Werner & Pfleiderer newsletter, Processing Lines, Vol. 9, no. 1, January 1999 suggests in FIG. 10 that fillers/fibers should be added after polymer has been melt-plasticated. When applicants, attempted to follow this prevailing teaching in the art and, added moist wood fiber to fluxed polymer, they surprisingly noted that under some conditions, that the steam pressure generated caused separation and forceful venting of wood fibers at vent ports. Vented steam and other volatiles arising from the composition at elevated temperature provided sufficient energy to interfere with melt processing.
In a variation of the process schematically shown in FIG. 1 of U.S. Pat. No. 5,518,677, applicants determined (contrary commonly accepted teaching, i.e., polymer first followed by wood fiber) that wet wood fiber could be added first and the bulk of the resulting steam vented from the process prior to adding, melting, and mixing of thermoplastic polymer to form a dense composite. U.S. Pat. Nos., 5,827,607 and 5,518,677 further suggest that composites can be prepared by heating the fluxed polymer to lower its viscosity thus improving wood fiber mixing and wetting processes, In so doing, all exterior surfaces of the fiber can be fully contacted by polymer and pores, crevice, cracks, passage ways, indentations, etc. become fully filled with polymer thus forming a composite having an exterior continuous polymer phase in which wood particles are dispersed as a discontinuous phase within the continuous thermoplastic phase. U.S. Pat. No. 5,518,677 further suggests that the temperature, pressure, and shear conditions during dispersive mixing must be sufficient to disrupt fiber cells and force polymer into the interior volume of the cells significantly increasing the density of the composite.
In the practice of this process technology, other process complications arise. Thermoplastic polymers particularly PVC composition, can thermally degrade at temperatures slightly above their melting points. This degradation is especially problematic when the products of the chemical reactions are toxic and corrosive. Such is the case with PVC where the degradation product is hydrochloric acid vapor (HCl). When moist biofiber is processed corrosive hydrochloric acid can be also produced when acid vapor combines with moisture. Since PVC decomposition proceeds by a free radical mechanism, decomposition can be autocatalytic especially in the presence of moisture, entrained oxygen (from fiber-trapped air), and hydrogen chloride. When polymer free radicals react, crosslinking can occur producing a rapid increase in melt viscosity and mixing torque. Gel Permeation chromatography reveals the effect of crosslinking in the form of broadened molecular weight distributions. Crosslinking of PVC reduces its utility in thermoplastic products and processes. Attempts to control this free-radical decomposition include addition of stabilizers most of which are free-radical scavengers. Recognizing this problem, many equipment manufacturers recommend that users of their equipment avoid production of PVC-biofiber composites unless special alloys are used in regions of the equipment where HCl production is likely to occur. Furthermore, upon experiencing an increase in mixing torque attributed to decomposition and crosslinking reactions, the manufacturers recommend shutting down the process before the viscosity increase causes the screw to seize in the extruder barrel, leading to an uncontrollable temperature rise with simultaneous evolution of significant and hazardous quantities of HCl.
Rauwendall, U.S. Pat. No. 4,798,473, describes the design of an extruder screw suitable for the continuous extrusion of various poly(ethylene) plastics that reduces induced power consumption and stock temperature generation. Poly(ethylene) polymers have a different melt flow character than PVC. Polyolefins are generally more stable and with similar molecular weights have lower viscosity at a defined temperature. Rauwendall mentions the severe problems that can occur when plastics having limited thermal stability, as for instance, poly(vinylchloride) (PVC) are processed at high screw induced stock temperatures.
Rauwendall, Polymer Extrusion, Hanser Publishing, 1994, p 529-32, discusses the calculation of the adiabatic and isothermal temperature rise expected in the narrow clearance gap between the barrel and top of the screw flights and concludes that the local temperature in gap can be widely vary in the range of from about 30.degree. C. to greater 150.degree. C. Rauwendall (Supra), p 398-99 describes multi-vent devolatilization and cascade devolatilzation processes for single screw extruders having various screw designs. He discusses the use of two extruders to devolatilize PVC which preferably passes through a stranding die and long vacuum transition box en route to a second single screw extruder. No mention is made of the pressures at which any of the vents operate.
Clearly, known combinations of equipment, processing parameters and material parameters suggest that optimizing process conditions, composite throughput and power consumption can be a complex undertaking. Some of process equipment parameters include barrel volume, temperature, temperature profile, pressure, shear rate, mass transfer, heat transfer, etc. Some of the material blend parameters include fluxed polymer stability, polymer rheology, fiber size and shape, moisture content, additive selection, etc.